This converter features contemporary units of length. There is also a special converter for historical units of length you might want to visit for ancient, medieval and other old units that are no longer used.

The American System (US Customary Units) and British Imperial Measures

American weights and measures are based on units used in Britain prior to 1826, when imperial system was officially established. Until 1960-1963 American and British units of length were different in 2 parts per million. In the middle of 20th century it started causing problems and the governments of the two countries agreed to refine the measures to exactly match. Since then Great Britain and the United States were using the same measures of length. In the U.S. the old standard of foot was retained with the name US survey foot.

From 1995 the UK adopted metric units for general use. The only imperial measures of length that can be officially used now are miles, yards, feet and inches for road traffic signs.

British Imperial Measure before 1963

The first English official measurement standards were defined in 15th century. British system of units, known as imperial units, was established in 1824. The British Weights and Meausurements Act of 1824 repealed all existing British measures legislation and redefined existing units. The 1963 Weights and Measures Act standartized the international measures.

International Nautical Measure

The international nautical mile was defined by the First International Extraordinary Hydrographic Conference, Monaco in 1929. This is the only definition in widespread current use, and is the one accepted by the International Bureau of Weights and Measures. Before 1929 different countries had different definitions, and the Soviet Union, the United Kingdom and the United States did not immediately accept the international value.

Optical

Geographical (German)

Computer Equipment

A rack unit, U or RU is a unit of measure that describes the height of equipment designed to mount in a 19-inch rack or a 23-inch rack. The 19-inch (482.6 mm) or 23-inch (584.2 mm) dimension refers to the width of the equipment mounting frame in the rack including the frame; the width of the equipment that can be mounted inside the rack is less.

Astronomical

Grace Hopper's units of distance

Grace Hopper is famous for her nanoseconds visual aid. People used to ask her why satellite communication took so long. She started handing out pieces of wire that were just under one foot long (11.80 inches) — the distance that light travels in one nanosecond. She also passed out packets of pepper, calling the individual grains of ground pepper picoseconds. She also used these aids to illustrate why computers had to be small to be fast.

Natural units

In physics, natural units are physical units of measurement based only on universal physical constants. The origin of their definition comes only from properties of nature and not from any human construct.

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